Prey-capture in the African clawed toad (Xenopus laevis): comparison of turning to visual and lateral line stimuli

Separately delivered visual and lateral line stimuli elicit similar but not identical orientation and approach by intact, sighted Xenopus. Response frequencies for visual stimuli declined sharply for distant or caudal stimuli while those for lateral line stimuli changed little. Turn angles correlated highly with stimulus angles but were smaller on average, so regression slopes were less than one. Regression slopes were smaller for visual than for lateral line stimuli, but this apparent difference was due to different distributions of stimulus distance interacting with the toad’s rotation center. Errors in final headings, most often under-rotations, did not differ by modality. Frequencies of lunges and arm capture movements were higher for visual stimuli both overall and especially for rostral proximal stimuli. The results demonstrate accurate orientation by sighted Xenopus to visual and lateral line stimuli; they are consistent with expectations based on in-register tectal maps. Orientation to lateral line stimuli is similar to previous results with blinded animals, revealing no heightened acuity in the latter. Modality differences indicate that the lateral line system is better for omnidirectional orientation and approach to distant stimuli whereas the visual system is more attuned to nearby rostral stimuli and more apt to mediate strikes.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Bias in the gradient-sensing response of chemotactic cells

We apply linear stability theory and perform perturbation studies to better characterize, and to generate new experimental predictions from, a model of chemotactic gradient sensing in eukaryotic cells. The model uses reaction-diffusion equations to describe 3(') phosphoinositide signaling and its regulation at the plasma membrane. It demonstrates a range of possible gradient-sensing mechanisms and captures such characteristic behaviors as strong polarization in response to static gradients, adaptation to differing mean levels of stimulus, and plasticity in response to changing gradients. An analysis of the stability of polarized steady-state solutions indicates that the model is most sensitive to off-axis perturbations. This biased sensitivity is also reflected in responses to localized external stimuli, and leads to a clear experimental prediction, namely, that a cell which is polarized in a background gradient will be most sensitive to transient point-source stimuli lying within a range of angles that are oblique with respect to the polarization axis. Stimuli at angles below this range will elicit responses whose directions overshoot the stimulus angle, while responses to stimuli applied at larger angles will undershoot the stimulus angle. We argue that such a bias is likely to be a general feature of gradient sensing in highly motile cells, particularly if they are optimized to respond to small gradients. Finally, an angular bias in gradient sensing might lead to preferred turn angles and zigzag movements of cells moving up chemotactic gradients, as has been noted under certain experimental conditions.     

Orientation responses of individual larder beetles,Dermestes ater (Coleoptera,Dermestidae), to directional shifts in wind stimuli

Anemotaxis in adult larder beetles, Dermestes ater,was investigated using a locomotion compensator, to uncover the mechanism(s) by which beetles maintain a course direction relative to a wind stimulus. Compared to walking in still air, anemotactically orienting beetles walk with the following characteristics over 60-s periods: (1) reduced locomotor and turning rates, (2) sustained, relatively straight paths with course directions at various angles to the wind, and (3) an increased tendency to stop for brief periods. Differences in wind speed affect mainly path straightness, which increases positively with stimulus intensity. Beetles track the wind direction equally well moving up or downwind, and they are able to orient at angles either close to the wind or at more oblique angles. When the wind direction was shifted 90°, the beetles turned, usually over the short angle, to their previous course heading relative to the stimulus. Indvidual beetles exhibited preferred course directions over several trials within a period of 20 min. Each beetle regained its particular anemotactic angle after the 90° shift in the stimulus direction. Although the beetles paused in some trials, stopping was not required to reorient to the altered stimulus direction.     

Permanent electric polarization and pyroelectric behaviour of the vertebrate skeleton

Summary Like the bones of the axial and the appendicular skeleton, the bones of the human skull are pyroelectric and hence piezoelectric, too. There are two directions of electric polarization, one at right angles to the other, which can be clearly distinguished by appropriate measurements.The first of these two directions of polarization in the cranial bones follows the longitudinal direction of the collagen fibres or the osteons which develop from them. This longitudinal polarization is particularly marked in the cranial bones of foetuses and children in their first years of life. Up to the 2nd or 3rd postnatal year the electrical axes inside the parietal bone and the frontal bone, for example, emanate like radii from a centre formed by the parietal tuber and the frontal tuber respectively, and extend right to the very periphery of the bone. The axes are aligned exactly parallel to the fibre striation of the bony tissue which is macroscopically visible at this age. After the 3rd year of life, this strictly radial orientation disappears and the electrical axes become adjusted to the partly new contours of the cranium.The second direction of polarization in the cranial bones persists through life. It goes from the external surface to the internal surface, i.e. it is at right angles to the lamellae and tissue layers of the bones. It is true that the sense of direction differs in lamina externa, diploë and lamina interna, but it never changes in the course of ontogenesis.Analogous electric polarization phenomena were found in the cranial bones which develop from connective tissue and those which are cartilaginous in origin; this was demonstrated in respect of the two types of bone forming the occipital bone. A comparative study of the dermatocranium and the various large scales of Acipenser sturio showed that the polarization pattern is similar to that found in our investigations of the human skull bones.Our findings in respect of the cranial bones are supplemented by a survey of the electric polarization pattern in the topographically adjacent structures, i.e. pericranium, galea aponeurotica and scalp and also the dura mater encephali.

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Zusammenfassung Die Knochen des menschlichen Craniums sind, wie die Knochen des Achsen- und Extremitätenskeletts, pyroelektrisch und damit auch piezoelektrisch. Es sind zwei aufeinander senkrecht stehende elektrische Polarisationsrichtungen vorhanden, die meßtechnisch klar unterscheidbar sind.Die eine Polarisationsrichtung der Schädelknochen besteht in Längsrichtung der Kollagenfasern bzw. der aus ihnen gebildeten Osteone. Diese Längspolarisation tritt in den Schädelknochen von Feten und von Kindern der ersten Entwicklungsjahre besonders deutlich in Erscheinung. Bis zum 2. oder 3. postnatalen Lebensjahr verlaufen die elektrischen Achsen z. B. innerhalb des Os parietale und des Os frontale wie Radien, vom Tuber parietale bzw. Tuber frontale als Zentrum ausgehend, bis zur äußersten Peripherie des Knochens. Der Achsenverlauf ist exakt parallel zu der in diesem Alter makroskopisch erkennbaren Faserstreifung des Knochengewebes. Nach dem 3. Lebensjahr verliert sich die streng radiale Orientierung und paßt sich den teilweise neu entstehenden Konturen des Craniums an.Die andere Polarisationsrichtung der Schädelknochen besteht zeitlebens in Richtung von ihrer Außenfläche zur Innenfläche, also senkrecht zu den vorhandenen Lamellen und Gewebeschichten. Ihr Richtungssinn ist in Tabula externa, Diploë und Tabula interna zwar unterschiedlich, ändert sich aber im Laufe der Ontogenese nicht.Die aus Bindegewebe entstehenden und die aus Knorpel präformierten Schädelknochen verhalten sich hinsichtlich der gefundenen elektrischen Polarisationserscheinungen analog, wie an den beiden Knochenanteilen des Os occipitale gezeigt werden konnte. Eine Vergleichsuntersuchung an dem Dermatocranium und an den einzelnen großen Knochenschuppen von Acipenser sturio ergab ähnliche elektrische Polarisationsverhältnisse wie in den untersuchten Knochen des menschlichen Craniums.Zur Ergänzung der Befunde an den Schädelknochen werden die elektrischen Polarisationsverhältnisse in den topographisch benachbarten Strukturen des Pericraniums, der Galea aponeurotica und der Kopfhaut, sowie der Dura mater encephali angegeben.

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With support from the Deutsche Forschungsgemeinschaft which is gratefully acknowledged.     

Point Me in the Right Direction: Same and Cross Category Visual Aftereffects to Directional Cues

Of the many hand gestures that we use in communication pointing is one of the most common and powerful in its role as a visual referent that directs joint attention. While numerous studies have examined the developmental trajectory of pointing production and comprehension, very little consideration has been given to adult visual perception of hand pointing gestures. Across two studies, we use a visual adaptation paradigm to explore the mechanisms underlying the perception of proto-declarative hand pointing. Twenty eight participants judged whether 3D modeled hands pointed, in depth, at or to the left or right of a target (test angles of 0°, 0.75° and 1.5° left and right) before and after adapting to either hands or arrows which pointed 10° to the right or left of the target. After adaptation, the perception of the pointing direction of the test hands shifted with respect to the adapted direction, revealing separate mechanisms for coding right and leftward pointing directions. While there were subtle yet significant differences in the strength of adaptation to hands and arrows, both cues gave rise to a similar pattern of aftereffects. The considerable cross category adaptation found when arrows were used as adapting stimuli and the asymmetry in aftereffects to left and right hands suggests that the adaptation aftereffects are likely driven by simple orientation cues, inherent in the morphological structure of the hand, and not dependent on the biological status of the hand pointing cue. This finding provides evidence in support of a common neural mechanism that processes these directional social cues, a mechanism that may be blind to the biological status of the stimulus category.     

Simultaneous tracking of flagellates in real time by image analysis

The hardware and software components are described to track many flagellates simultaneously in real time. The technique is based on on-line digitization of four frames taken at 80 ms intervals and stored in a specialized video memory. The outline and position of each organism are determined by chain coding and followed through the recorded series of images. The movement vectors of all organisms tracked are stored in the form of deviation angles from a predefined stimulus (light or gravity) direction and the distance each organism has moved in the time interval determined by the hardware clock of the computer. Subsequent programs allow one to determine circular histograms of movement directions and velocities in dependence of the movement direction. Examples of several orientation patterns are given for both photoorientation and gravitaxis in the dinoflagellate, Peridinium gatunense.Supported by grants from the Deutsche Forschungsgemeinschaft (SFB 305, Marburg and Ha 985/6-1, Erlangen)     

Orientation by helical motion—III. Microorganisms can orient to stimuli by changing the direction of their rotational velocity

Organisms that move along helical trajectories change their net direction of motion largely by changing the direction, with respect to the body of the organism, of their rotational velocity (Crenshaw and Edelstein-Keshet, 1993,Bull. math. Biol. 55, 213–230). This paper demonstrates that an organism orients to a stimulus field, such as a chemical concentration gradient or a ray of light, if the components of its rotational velocity, with respect to the, body of the organism, are simple functions of the stimulus intensity encountered by the organism. For example, an organism can orient to a chemical concentration gradient if the rate at which it rotates around its anterior-posterior axis is proportional to the chemical concentration it encounters. Such an orientation can be either positive or negative. Furthermore, it is true taxis—orientation of the axis of helical motion is direct. It is neither a kinesis nor a phobic response—there is no random component to this mechanism of orientation.     

Hydrodynamic orientation of crayfish (Procambarus clarkii) to swimming fish prey

Reversibly blindfolded crayfish (Procambarus clarkii) react to small swimming fish (Astyanax fasciatus mexicanus) approaching or passing nearby with antennal and cheliped movements and body turns (Fig. 3). We studied the accuracy and dynamics of crayfish orientation responses to the previously analyzed hydrodynamic disturbances caused by the fish, mostly produced by tail flicks.Antennal and cheliped movements started slightly before the onset of turning responses (Fig. 4). Antennal sweeps were performed most rapidly. 50% of the appendage sweeps resulted in contacts with the fish (Fig. 5).Most turns were directed toward the stimulus (Fig. 6). Response amplitudes increased with increasing stimulus angle. Turns were accurate for small stimulus angles, but smaller than expected for larger ones. Sweeps of ipsilateral antennae and chelipeds were generally directed backwards, while those of contralateral appendages were smaller and directed forwards. The amplitudes of appendage sweeps first increased with increasing stimulus angle and then decreased again for more caudal stimulus directions. Lateral stimuli (60°–120°) from opposite sides were usually significantly distinguished. The amplitudes of the different elements of orientation behaviour were highly correlated with each other, indicating that they were directed by the same sensory input.     

DETERMINATION OF RHIZOID ORIENTATION BY LIGHT AND DARKNESS IN GERMINATING SPORES OF ONOCLEA SENSIBILIS

When spores of the fern, Onoclea sensibilis L., are allowed to germinate in darkness, the rhizoid and the protonema are positioned at close to a right angle. If the spores are exposed initially to light and allowed to germinate, the rhizoid and protonema are positioned nearly axially, at opposite ends of the spore. The greater the duration and intensity of the initial illumination, the greater the tendency towards axial arrangement. All colors of light are active to some degree, and the effects are intensity-dependent. The response occurs in a uniform light field and is not dependent on a directional stimulus; the phenomenon reflects the relative arrangement of one part of the gametophyte to another part but not the orientation of growth with respect to an external stimulus. Direct tests show that neither the relative rhizoid orientation nor initial polarity of germination are affected by unilateral white light or polarized red light; the subsequent growth of the protonema, however, is oriented perpendicular to the plane of light polarization. The effects of light in determining the positional relationship between rhizoid and protonema are interpreted in terms of a hypothesis proposing light-induced changes in the structure and mechanical properties of the spore wall.     

Population differentiation of migratory directions in birds: comparison between ringing results and orientation behaviour of hand-raised migrants

Summary Blackcaps (Sylvia atricapilla) from five areas in Central Europe were hand-raised. Their autumn migratory orientation was tested in funnel-shaped cages. Their directional choices were compared to recoveries of conspecifics ringed during the breeding season in the same areas, which are situated on a transect across a migratory divide between southeastward and southwestward migrating populations. Results from ringing and orientation tests were in good agreement with respect to mean direction and dispersion of flight directions. An exception is the area around Linz (NW Austria), right on the migratory divide, where ringing yielded a strong scatter, but hand-raised birds chose westerly directions. The recent establishment of a novel migration route toward the British Isles was reflected in both data sets: in southern Germany the percentage of northwestward migrants is 6.8% according to orientation tests and 11.8% according to ringing recoveries. Testing the orientation of young passerines in captivity can yield valuable information about population differentiation of migratory behaviour. It is more efficient than ringing in this respect, because it circumvents the low recovery rates and is free of biases affecting ringing data. In the blackcap, geographic differentiation of migratory directions occurs on a finer scale than previously recognized and can change significantly within 2–3 decades.     

Peculiarities of Visually Sensitive Neurons of the Extrastriate Associative Area 21b of the Cat Brain Cortex

On cats with pretrigeminal brainstem transection, we studied the properties of visually sensitive neurons of the extrastriate associative cortical area 21b. The dimensions and spatial distribution of the receptive fields (RF) of the neurons within the vision field were determined. It was found that large-sized RF prevailed within the area 21b (10 to 200 deg², 61%; greater than 200 deg², 22%), whereas small-sized RF (1 to 10 deg²) constituted 17% of all the studied RF. Stationary visual stimuli evoked on–off, off, and on responses in 43, 30, and 27% neurons of the area 21b, respectively. In the cases where moving stimuli were presented, 35% of the neurons demonstrated directional sensitivity; the rest of the neurons (65%) were directionally insensitive. We also found a group of neurons that were capable of differentiating not only the direction of the stimulus movement along the RF but also the dimension, shape, and orientation of a complicated moving stimulus. Taking into account the data obtained, we discuss the functional role of the neurons, which demonstrated a specific (specialized with respect to a set of the parameters of visual stimulus, and not to a single parameter) response in central processing of the sensory information.     

Perception of polarization plane,colour and movement in two dimensions by the crab,Carcinus

Summary The spectral sensitivity of movement perception in Carcinus has a peak near 508 m, irrespective of the direction of the movement or of the plane of polarization of the stimulating light, although the primary sensory cells are 10 times as sensitive to light in one plane of polarization as in the plane at right angles. This balance in the input to the movement perception system breaks down with a stimulating light at <439 and at >650 m. Only one set of connexions between the primary receptors in the movement perception system agrees with all the results of manipulating the various attributes of the visual input, and this model has unique properties which allow the subsequent discrimination of each attribute of the stimulus independent of the other attributes.I would like to thank Robert Santer for his assistance with the numerous experimental observations which form the basis of this paper.     

Crystallographic characterization of the crossed lamellar structure in the bivalve Meretrix lamarckii using electron beam techniques

To understand the formation mechanism of crossed lamellar structures in molluskan shells, the crystallographic structural features in the shell of a bivalve, Meretrix lamarckii, were investigated using scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy with a focused ion beam sample preparation technique. Approximately 0.5 μm-thick lamellae (the second-order units) are piled up obliquely toward the growth direction to form the first-order unit and the obliquity is inverted between adjacent units along the shell thickness direction. The first-order units originate around the center of the shell, initially growing parallel to the shell and subsequently curving toward the inner or outer surfaces. The lamellae consist of aragonite granular and columnar layers, which group together to adopt the same crystal orientation forming crystallographic units (crystallites). Multiple {1 1 0} twins are common both in the granular and columnar layers. The crystallite c-axis is parallel to the columns and is inclined at angles 0–50° from the lamellar normal (dispersing among individual lamellae), toward the shell growth direction. Probably, the directions of the a- and b-axes are random in the lamellae, showing no specific orientation.     

Discrimination of a Right Angle with Monocular and Cyclopic Perception

Psychophysical tests with monocular and cyclopic perception were carried out to evaluate the accuracy of discrimination of right, acute, and obtuse angles. Tests with monocular perception were carried out with stimuli made by light line segments, spots, or elements of the Oppel-Kundt and Muller-Lyer figures. In tests with cyclopic perception, pairs of V-shaped stimuli with an identical orientation in the visual field and equal length of the sides but different divergence angles were presented to the different eyes of subjects. The test data demonstrated features of the perception of a right angle, namely, a high accuracy of reproduction, periodicity of errors as a function of the general orientation of a stimulus, similar characteristics of the manifestation of geometric illusions in angle reproduction and length comparison, and the manifestation of Hering’s law in cyclopic perception. These results agree with the multilocal hypothesis, which explains the perception of right and other angles on the basis of the information about the coordinates of stimulus parts.__________Translated from Fiziologiya Cheloveka, Vol. 31, No. 4, 2005, pp. 14–26.Original Russian Text Copyright © 2005 by Bulatov, Bertulis, Bulatova.     

Combined effects of speed and directional change on postural adjustments during gait initiation

The study of gait initiation (GI) has primarily focused on gait initiated in a forward direction, however, in everyday life, GI is often combined with a directional change. Ten young adults initiated gait with their right foot in four directions (to the left: −15°, straight ahead: 0°, to the right: 15° and 30°) at self-selected and fast gait speeds. The relationship between starting direction of GI and the lateral center of foot pressure displacement for normal (r² = 0.57) and fast gait speed (r² = 0.75) indicated that the lateral component plays an important role with regards to controlling the desired direction of gait. At the first step of the swing limb, the progression velocity of the center of mass (CM) remained slower for the 30° condition only, whereas no difference was found between directions for CM velocity perpendicular to the intended direction. These results suggest that postural adjustments are scaled to initiate gait in a predetermined direction. By the first step, the orientation of CM is toward the intended direction of gait, however, when gait is initiated in combination with a large change in direction, additional adjustments may be required to reach the intended progression velocity.     

Visual guidance of predatory attack by a scincid lizard, Eumeces laticeps

Visual stimuli are important determinants of the exact placement of predatory bites by the broad-headed skink (Eumeces laticeps), a member of a varied group of lizards known for their chemoreceptive abilities, the Autarchoglossa. The skinks preferentially attack large larval bess beetles (Popilius disjunctus) just posterior to the head and use visual cues to identify the attack site. Head coloration or contrast between the dark head and light anterior thorax releases attack just behind the edge of the darkly coloured region. Skinks also employ a directional cue to guide attacks to the postcephalic region. This directional stimulus is anterior position with respect to the prey's direction of motion, which is a fairly reliable indicator of head location even in the absence of distinctive head colour. When colour/contrast and directional cues disagree, the end of the prey bearing head-colour stimuli is selectively attacked if the colour cues are strong, but both ends are attacked with nearly equal frequency when coloration is less obvious. An artificially large and brightly painted head appears to function as a supernormal releasing stimulus for attack.     

Developmental responses of portulaca seedlings to conflicting spectral signals

Summary Portulaca oleracea seedlings avoid growing in the direction of neighbouring plants even when they are very small or remote. The present study was designed to determine the relative effect on the development of Portulaca seedlings of light availability (i.e. the resource level) as compared with spectral composition (i.e. the signal of future competition for the resource). The plants were subjected to various intensities of photosynthetic light and red/far-red (R/FR) ratios from opposite directions. The seedlings became recumbent preferentially towards the direction with the lower FR light, even when this meant growing towards plastic that absorbed 20 times more photosynthetic light. A preference for the direction with higher photosynthetic light over lower FR was also found, though only under extreme light differences. The response of the seedlings was not absolute: the orientation chosen depended on the light received from other alternative directions.     

Analysis of surface wave direction by the lateral line system of Xenopus: Source localization before and after inactivation of different parts of the lateral line

The turning responses of clawed toads (Xenopus laevis) to surface waves were examined in animals with an intact lateral line or with different combinations of lateral lines reversibly inactivated by CoCl₂. The responses were characterized with respect to response frequency, turning accuracy, turning side, response time, and swim distance. After the inactivation most animals still responded to surface waves but the responses were different from those of animals with an intact lateral line. They also differed according to the combination of inactivated lines. In all experiments the responses for stimuli in some sectors of the surface did not differ from controls. The location of these sectors co-varied with the position of the intact lines, i.e., normal responses were found for frontal stimulus directions when head lines were intact and for caudolateral stimulus directions when trunk lines were intact. Their size was larger when lines on both sides of the body were intact and smaller when only lines on one side were intact. When the number of functional lines was reduced to one or two on one side of the body the turning angles shown within the sector of normal responses were maintained for stimulus directions outside these sectors. These results can be interpreted as indicating that head and trunk lines represent different position values. When only a single line was functional the toads still turned towards the stimulus source more often than by chance.It is hypothesized that Xenopus uses two mechanisms to determine the direction of surface waves. One uses the position values of head and trunk lines; this mechanism is comparable to the place value postulated for individual head neuromasts of surface feeding fish. The other uses the information encoded in the activity pattern that is elicited in one line when the surface wave travels over the line. This second mechanism yields information about stimulus side but not about stimulus angle.     

Directional dependence of osteoblastic calcium response to mechanical stimuli

In adaptive bone remodeling, mechanical signals such as stress/strain caused by loading/deformation are believed to play important roles as regulators of the process in which osteoclastic resorption and osteoblastic formation are coordinated under a local mechanical environment. The mechanism by which cells sense and transduce mechanical signals to the intracellular biochemical signaling cascade is still unclear, however to address this issue, the present study investigated the characteristic response of a single osteoblastic cell, MC3T3-E1, to a well-defined mechanical stimulus and the involvement of the cytoskeletal actin fiber structure in the mechanotransduction pathway. First, by mechanically perturbing to a single cell using a microneedle, a change in the intracellular calcium ion concentration [Ca²⁺]_i was observed as a primal signaling response to a mechanical stimulus, and the threshold value of the perturbation as the mechanical stimulus was evaluated quantitatively. Second, to study directional dependence of the response to the mechanical stimulus, the effect of actin fiber orientation on the threshold value of the calcium response was investigated at various magnitudes and directions of the stimulus. It was found that the osteoblastic response to the perturbation exhibited a directional dependence. That is, the sensitivity of osteoblastic cells to a mechanical stimulus depends on the angle of the applied deformation with respect to the cytoskeletal actin fiber orientation. This finding is phenomenological evidence that cytoskeletal actin fiber structures are involved in the mechanotransduction mechanism, which may be related to cell polarization behaviors such as cellular alignment caused by mechanical stimulation.     

Conformational studies of peptides. The crystal structures of N-acetyl-L-prolinamide and N-acetyl-(S)-thiazolidine-4-carboxamide

The crystal structure of N′-acetyl-L -prolinamide and its isomorphous alalog N′-acetyl-(S)-thiazolidine-4-carboxamide was determined using highly accurate parameters obtained by room- and low-temperature data-collecting systems. Both crystals are orthorhombic, space group P2₁2₁2₁, with four molecules per unit cell held together by a hydrogen-bond system that extends in all directions. Both molecules exhibit the following conformational features: the acetyl group is in the trans configuration in respect to the tertiary amide. The primary amide is almost at right angles with respect to the mean plane of the ring and the –NH₂ group is over the ring. The pyrrolidine and thiazolidine rings were found to be rather flexible with C^β puckering in the former and sulfur in the latter.